When the Ordo Malleus sends in a force to purge a planet from Daemons or even sends an attack force into the warp, the Inquisitor in charge can create the bulk of his force using Grey Knights or Inquisitorial Storm Troopers, or even a combination of both. Each of these squads has their own roles in the Daemonhunter army and can be customized to fit one in particular or try to cover several at once. While the Grey Knight troop choice is the much more elite of the two, the Storm Troopers are much larger in number and much more expendable, at least compared to the sacred Space Marines of the Chapter 666. When additional bodies are needed, an Inquisitor will often fill up his space with Storm Troopers, as the number of Grey Knights may be insufficient.In an army focusing on Grey Knights, you will probably be taking at least 3 Grey Knight squads in a game at or above 1500 points, though it is possible to only take two. Anyways, you will have to try Grey Knights serve most of the purposes your troops can take up. They will score for you, they will act as counter-assault or even assault units, they will be able to kill some of the lower AV transports, and try to take out hordes of enemies. So you will have to equip them well. The strongest strength you can get on these guys, however, is 6. This applies to both in the assault and shooting with Psycannons. A strength of 6 is a bit too low and too unreliable to kill most transports, though, so that is where Inquisitorial Storm Troopers can help. You can equip them with meltaguns, and they are very cheap, so they won't put a large dent in your points. I'm going to do a short pros and cons with the units, and then I'll go into more details.

Grey Knights
Pros:

High WS

Can put out a lot of shots

Can use Psycannons/Incinerators

Great at counter-assaulting

Above average survivability as it comes to troops

Fearless and other Grey Knight rules

Very versatile

Cons:

Very expensive

Aren't great at everything

Cannot kill most vehicles

Cannot buy transports

Inquisitorial Stormtroopers
Pros:

Very cheap

Can put out a lot of shots

Can be equipped with a variety of weapons

Can buy transports - More mobility and survivability

Sergeant is optional and can buy weapons out of armory

Cons:

Terrible in the assault

Low strength on most shots

Weapons taken are very specialized

Transports are overpriced and cannot take other units for a ride

So you can see why you might want to take both Inquisitorial Stormtroopers and Grey Knights. The Grey Knights protect the Stormtroopers while they both lay down a lot of fire, and the Stormtroopers move out and try to meltagun a transport or monstrous creature, etc.

But there are other options for a Grey Knight army. Grey Knights can induct Imperial Guard allies, bringing in an Infantry Platoon. Infantry Platoons are 3 squads of guardsmen, not attached to other (though you can combine them if you wish), consisting of, at a minimum, 1 Platoon Command Squad (1 Platoon Commander and 4 Guardsmen) that can issue Imperial Guard commands and two Guardsmen squad consisting of 10 guardsmen each. The Platoon squad can have as many flamers, meltaguns, or plasma guns as you want, while the two other squads can only have one. Coming at a minimum of 130 points, you can attach a Chimera to each squad and equip them with meltaguns. The Chimeras you give them are only 55 points, compared to the 85 points for one in the Daemonhunter's book, and have more firing points. These Chimeras can also carry any unit, but can only deploy with the squad it was purchased for. This means you can deploy them empty and have Grey Knights jump in for a ride or to keep them out of fire for a while. The infantry platoons can either ride in a chimera or hug objectives for you, allowing your Grey Knights to be the strike force of your army. Just be careful, as doing this adds quite a lot of extra kill points for your opponent, but you can combine your two guardsmen squads if you would like to in order to reduce the number of kill points. Note that you could hypothetically take a Leman Russ if you included two Infantry Platoons, but chances are you will never have that many points to spare and you might as well be playing Imperial Guard with allied Daemonhunters. (Hm, I wouldn't mind Straken leading some Grey Knights... Counter-attack and Furious Charge would be delightful.)

So, now you know what options are out there. But it is still probably too much to sort through, so let me give you some sample Troop Choice layouts that I use.

(Note: I almost never use mini-purgation squads. If you do, I recommend taking them as Fast Attack to allow them to Deep Strike, but not score, as there are too few wounds to attempt to score an objective)

Inquisitorial Meltagun Squadron - 125
2 Meltagun Stormtroopers, 3 Stormtroopers, 1 Rhino w/ Extra Armor
(Note: I use this to try to zoom up to a vehicle I need killed and fire at it, probably losing both squads in the process)

I'll try to do a separate article on how to use Allies if you choose to, so I will cover some of my common ideas for Imperial Guard Platoons there. Below I'm going to list a combination that I field often, referring to each unit as the name given above.

In my lists, I usually want spend somewhere between 510 and 545 points on Land Raiders and Dreadnoughts, so you can see why I often don't include the Meltagun squadrons in lower point cost games as I don't even have enough points for my long range anti-tank goodies from Heavy Support. At 2000 points, however, I will usually either take an Inquisitorial Meltagun Objective Squadron or an Assassin, depending on what I want to try out that day. Another thing you should note is that when Inquisitorial Stormtroopers are in a Rhino, it is considered open-topped, so you have to be aware of that. It allows a bit more shooting and a lower strength on an explosion, but it makes losing the Rhino that much more likely.

I hope this overview on troops helped you get an idea on how to field Daemonhunters. Also, this is how I play, and not the only way, so go with what you want to do! Like Incinerators more than Psycannons, take two squads of Incinerators and one Psycannon. Or maybe you really like the smaller squads, so field them. Everything is how you want to play them, and my way isn't always the most competitive. Anyways, next time I'll either do a post focusing on the Justicar or I'll move to the final unit type overview, the Heavy Support choices.